Die casting is the process of producing molded parts by forcing molten metal under high pressure into a molding cavity. The molding cavities are cavities created using, for example, two hardened steel dies called die blocks. The first die block may be called the ejector die block and can contain the ejector pins. The second die block may be called the cover die block and is usually secured to a stationary portion of the die casting machine. Both die blocks meet at a parting line. The metal alloy is poured into an injection sleeve, and from that injection sleeve, the metal alloy travels in response to a given pressure into molding cavities to take a desired shape.
In some conventional machines, once the metal alloy is pushed into the molding cavities, hardening and taking shape within the molding cavities, the molded parts may then slide off the cover die block and remain on the ejector die block. The ejector die block has ejector pins used to push the molded parts or the castings out of the ejector die block. The ejector pin plate found in the ejector die block is used to drive all of the ejector pins in the ejector die block at the same time and with the same force. The ejector pin plate may then retract the ejector pins in order to regain position for the next casting.
The die casting process normally involves first spraying the injection sleeve with a lubricant to prevent the metal alloy from sticking to the injection sleeve. A predetermined quantity of molten metal is ladled into the injection sleeve. The molten metal is then forced under high pressure from the injection sleeve into the molding cavities using a drive piston, the high pressure maintained until the castings solidify. The die blocks are then separated and the castings are ejected from the die blocks using the ejector pins. The scrap metal alloy, remaining in the runners (channels connected to the molding cavities), the gate, sprues and flash are then separated from the castings.
A problem in die casting is the porosity of the metal of the die casted parts resulting from the entrapment of air from above the liquid metal in the injection sleeve. This may be caused at least in part by the velocity of the hammer or the plunger for exerting pressure upon the metal alloy and the turbulent nature of the molten alloy. Another problem leading to a decrease in strength or durability of the die casted parts is the entrapment of contaminants, such as the lubricant used to coat the injection sleeve, within the metal alloy. The contaminants weaken the lattice structure of the metal as it solidifies and renders the die casted parts more brittle. To compensate for such structural weaknesses, it is typical to design parts to use more metal to maintain a target strength.
A solution to the entrapment of air in the die casted parts is the use of a die casting machine with a full injection sleeve and a counter plunger moving in the injection sleeve during a first stage of the injection. The counter plunger is placed in the movable die block. At the moment of die closing the counter plunger is pushed forward, and used with a plug closing a fill port at the opposite end of the injection sleeve. This use of the counter plunger with the plug allows for the complete filling of at least a portion of the injection sleeve with the metal alloy, leaving no room for air in the portion of the injection sleeve before the metal alloy is hammered into the molding cavities. However, this particular solution does not address the issues of the contaminants, such as the lubricant, mixed within the metal alloy, which may affect the structural integrity and durability of the die casted parts. However, this solution does not effectively separate the contaminant-heavy liquid metal from the rest of liquid metal.